Electrolytic processes for the production of magnesium metal are generally based upon the reduction of magnesium chloride resulting in the generation of magnesium metal and chlorine. When a cell is fed with a very pure magnesium chloride, particularly when free from such impurities as iron, boron, nickel, copper, sulfates, magnesium oxide and water, it is relatively easy to design a cell which operates very efficiently and which produces high quality magnesium and chlorine gas, both of which have high market value. As the quantity of impurities in the magnesium chloride fed to the cells is increased the design and operation of the cell become more complex, the efficiency of the cell is lowered and the quality of the products suffers. Accordingly, practical cell designs in production use today reflect the compromises required by impurities in the magnesium chloride cell feed.
Some impurities in cell feed, such as iron, boron, nickel, copper and sulfates, are easily controllable by conventional chemical engineering methods. The preparation of inexpensive magnesium chloride cell feed, however, which is free from oxygen and hydrogen containing impurity has eluded the industry, despite intensive research for more than 50 years.
Most of the magnesium chloride cell feed used in the production of electrolytic magnesium is obtained from sea water. In order to use sea water as a source of magnesium the usual procedure is to treat the sea water with an alkali in order to concentrate the magnesium component as a hydroxide which is a solid and can, therefore, be separated easily from the sea water. In this process, however, the chlorine produced in the electrolysis of the magnesium chloride cell feed must be used in converting the hydroxide into magnesium chloride and therefore does not result in the production of saleable chlorine.
An objective of the industry for many decades has been the production of anhydrous and magnesium oxide free magnesium chloride cell feed by the dehydration of concentrated brines, either naturally occurring, such as from the Great Salt Lake in Utah, or from the bitterns of other salt processing operations, such as from potash production. Industry, however, has not been successful in finding a way to commercially dehydrate brine to obtain completely anhydrous magnesium chloride cell feed free of magnesium oxide. The reason is that the chemical equilibria of the reactions which occur during the dehydration of magnesium chloride brine are such that hydrolysis occurs and a portion of the magnesium chloride is converted to magnesium oxide, particularly during the last stage of dehydration. The hydrolysis reactions can be inhibited if the dehydration is performed in an atmosphere of certain chlorine compounds, such as hydrogen chloride or phosgene. The principal economic and technical difficulties associated with such reagents result from their aggressiveness to available engineering materials of construction and the hazards involved in their use.
Another method of inhibiting hydrolysis during dehydration of magnesium chloride is to dry the magnesium chloride under non-equilibrium conditions such as by the use of commercially available spray dryer equipment. Those skilled in the spray drying of magnesium chloride brine are able to produce magnesium chloride cell feed containing no more than 5% each of magnesium oxide and water. Magnesium chloride cell feed containing this range of impurities, however, normally causes passivation of cell cathodes, reduced cell efficiency and the generation of a large amount of cell sludge.
Removing cell sludge is costly. In addition, cell sludge entraps metal, thereby reducing the overall efficiency of the cell and if the sludge accumulation is not timely removed, it will accumulate sufficiently to short out the electrodes.
Therefore, it is customary to practice further purification before it can be used as a practical cell feed.
Such purification can be accomplished by the chlorination or phosgenation of the remelted spray dried magnesium chloride powder, which eliminates the water and converts the magnesium oxide impurity to magnesium chloride. The efficiency of these reactions is relatively low, resulting in the excessive use of chlorine or phosgene. Equipment costs and hazards are high. Accordingly, this method has not yet been reduced to industrial scale practice.
Another method of purifying magnesium chloride cell feed is to (1) melt the impure powder, to (2) remove the contained water by chemical dehydrogenation using a suitable gas or by reaction with a suitable metal, and to (3) physically remove the solid magnesium oxide by settling and decanting the purified magnesium chloride liquor, or by the use of centrigues or filtering devices. This method has both technical and economic disadvantages. The particles of magnesium oxide impurity are usually submicron to micron in size and thus are extremely difficult to remove from liquid magnesium chloride. The very small particles of magnesium oxide which are then carried into the cell with the feed have been found to be the most troublesome form of magnesium oxide in causing cathode passivation and reduced cell efficiency. Moreover, during the melting of the spray dried powder additional hydrolysis occurs, forming additional magnesium oxide. During the physical separation of the magnesium oxide from the purified magnesium chloride substantial amounts of magnesium chloride are entrapped within the interstices of the magnesium oxide, resulting in excessive loss of the valuable magnesium chloride feed in the waste sludge. The high cost of dehydrogenation of the melt, either by the use of a purging gas at elevated temperature or by the use of a metal which acts as a scavenger increases the serious economic problems associated with this purification procedure.
This invention is directed to a method for purifying spray dried magnesium chloride of its water and magnesium oxide impurities which is economical and avoids the technical difficulties of the other purification schemes described briefly above. This method comprises the in situ final purification of the spray dried magnesium chloride within the electrolytic cell itself by appropriate cell design and operation and the feeding of the spray dried powder having preselected physical and chemical ahcaracteirstics directly to the cell in the proper manner.
Feeding a magnesium electrolytic cell with an unmelted granular feed is not new to the art and is referred to in the nomenclature of the trade as "solid feed." At least one commercial producer feeds its magnesium cells with a coarse granular magnesium chloride feed containing typically 20% water impurity. However, the resulting chlorine coproduct of this cell is seriously contaminated with a large amount of hydrogen chloride and large quantities of sludge are produced. In times of emergency operators of the I. G. Farben type of magnesium electrolysis cell have been known to dry charge coarse feed directly to the cell, but such dry charging is not commonly practiced because of operational difficulties and poor economics.
This invention relates to the discovery that when powdered spray dried magnesium chloride feed containing small amounts of hydrogen and oxygen containing impurities is fed under the proper conditions to a cell of suitable design that: (1) the magnesium chloride powder melts instantaneously, with the evaporation of a substantial portion of the hydrogen containing impurities, which then is immediately removable from the cell (2) hydrolysis of the swiftly melted powder is substantially eliminated by its speed of melting and its immediate entrainment in the circulating electrolyte; (3) the impure feed is carried by the circulating electrolyte immediately into the anode-cathode space, before the impurities have had an opportunity to settle to the bottom of the cell; (4) substantially all of the magnesium oxide particles and the remaining water are chlorinated in the anode-cathode space by the cell chlorine produced at the anode; and (5) superior cell performance is obtained, with the production of high quality chlorine co-product and an extremely high magnesium ion efficiency is achieved.